At use of Circuit Switched FallBack, CSFB, or Short Message Service, SMS, over SGs, in a cellular system, a Mobility Management Entity, MME, Packet Switched, PS,-register a User Equipment, UE, and the MME also performs signalling via interface SGs, towards a Mobile Switching Centre, MSC/VLR, to trigger the MSC/VLR to CS-register the UE.
At a Mobile Terminated, MT, call the MSC/VLR informs/instructs the MME via SGs-signalling that the UE has an incoming call, and the MME in turn instructs the UE to perform an access and domain change to the CS access to enable the UE to take the call in the CS access.
An MT call, or an MT SMS, will not succeed e.g. when there is an issue with the communication via SGs or when there is an issue with the MME where the UE is registered. This is addressed by standard.
In 3GPP TS 23.007 chapter 26 a method is described to handle the case when the MSC/VLR where the UE is registered is not able to contact the MME where the UE is registered. The reason for this failure may be either an issue with the communication via SGs or an issue with the MME where the UE is registered. Multiple MME are in this case aggregated in an MME pool. In this situation the MSC/VLR has, according to the standard, an option to send a paging request to an alternative MME in same MME pool.